1. Field of the Invention
The present invention relates to tool holders including a tool holder main body and a tapered collet, for chucking a shank portion of a cutting tool.
2. Description of the Background Art
The applicant of the present invention already proposed tool holders having a structure in which a drawing screw is rotatably attached to a tool holder main body, and this drawing screw draws a collet deep into a collet holding hole, in Japanese Patent Publication Nos. H08-90318 and 2008-30138 of unexamined applications.
The tool holders described in Japanese Patent Publication Nos. H08-90318 and 2008-30138 of unexamined applications include a tapered collet for inserting a shank portion of a tool therein, and a drawing screw that is screwed in the tapered collet. A wrench is inserted from a rear end opening of a pull stud attachment hole, and the drawing screw is turned in a tightening direction to draw the tapered collet deep into a collet holding hole, thereby chucking the tool inserted in a tool insertion hole of the collet.
Incidentally, when manufacturing and assembling such a conventional tool holder, a drilling process for the drawing screw is performed from the rear end of a shank portion, as in a drilling process for the pull stud attachment hole. Then, when assembling the tool holder, a slip seat, such as a washer, is inserted from a rear end opening of the pull stud attachment hole so as to adjoin an annular stepped portion, which is formed at the top end of a hole located on the top end side of the pull stud attachment hole. Thereafter, the drawing screw, which has a shaft having a smaller diameter and having external threads formed therein, and a head having a larger diameter, is inserted from the rear end opening of the pull stud attachment hole. Specifically, the drawing screw is inserted with the shaft facing the top end and the head facing the rear end, so that the head adjoins the rear end of the slip seat. At this time, the shaft extends through a central hole of the slip seat, and is screwed in the rear end of the tapered collet. Then, a stopper member is inserted from the rear end opening of the pull stud attachment hole, and is screwed on internal threads formed in the inner peripheral surface of the pull stud attachment hole. The stopper member fixed in the pull stud attachment hole faces the rear end face of the head of the drawing screw to prevent rearward movement of the drawing screw. The front end face of the head of the drawing screw is received by the top end of the hole located on the top end side of the pull stud attachment hole, via the slip seat. Thus, the drawing screw is rotatably positioned with a small sliding resistance, while limiting forward movement of the drawing screw.
However, the inventors found that there are some things that should be improved in such conventional tool holders. That is, the stopper member can be loosened and can fail to serve as a stopper member for the drawing screw.
Moreover, if the tool length of the tool holder, which is a distance from the top end of the shank portion formed along the outer periphery of the rear end of the tool holder to the top end of the tool holder, is longer than that of typical tool holders, the use of a short drawing screw requires a deep hole drilling process to form a hole from the rear end of the shank portion, which increases the processing time and the processing cost. If the same drilling process for the drawing screw is used as that used in tool holders having a short tool length, the length of the drawing screw is increased, thereby reducing rigidity of the drawing screw. Thus, the tapered collet can be drawn obliquely.